True eels are teleostfish. They have long and narrow bodies like snake. Adult eels can be as short as 10 cm or as long as 3 m. It depends on their species. The large eels can weigh up to 65 kg.

Eels have fewer fins than other fish. They do not have all the belly and chest fins. The back and anal fins are long and usually connected to the tail fin. The fins do not have spines.

The shoulder bones are separate from the skull. The scales have smooth edges or are absent.

Eels hatch from eggs. Baby (larval) eels are flat and transparent (clear). They are called leptocephalus (Greek for "thin head"). A young eel is called an elver. For a long time people did not know where eels came from, because baby eels look very different from adults. They thought the babies were a different species.

Most eels prefer to live in the shallowest parts of the ocean. They live at the bottom of the ocean, sometimes in holes. Eels in the Anguillidae family come to fresh water to live there. Eels in the Nemichthyidae family swim about 500 m below the ocean surface. Eels in the Synaphobranchidae family live as deep as 4000 m below the surface.